1. Field of the Invention
The present invention relates to a backlight device including a backlight module including a plurality of LED chains each being formed of light emitting diodes (LEDs) connected in series and a backlight driver integrated circuit (IC) for driving the plurality of LED chains, and to a display device including the backlight device and a display panel for displaying video by adjusting the transmittance of light from a light source.
2. Description of the Related Art
In recent years, thinning of a liquid crystal display device using a liquid crystal display is advancing. FIG. 1 is a schematic block diagram of a liquid crystal display device. The liquid crystal display device mainly includes four modules. The first module is a liquid crystal display panel module 104 formed of two glass substrates sealing liquid crystal therebetween, in which a voltage is supplied to the liquid crystal so as to change inclination of the liquid crystal molecules, to thereby change the transmittance of light (modulation degree of light passing through the liquid crystal). In the liquid crystal display panel module 104, liquid crystal cells forming pixels are aligned two-dimensionally, and the liquid crystal cells are each sequentially controlled so that the transmittance of light may be changed two-dimensionally.
A panel driver 107 is a module for controlling the liquid crystal display panel module 104. The panel driver 107 synchronizes and outputs display data to the liquid crystal display panel module 104.
The third module is a backlight module 100, which is disposed on a rear surface of the liquid crystal display panel module 104 so as to be used as a light source for applying illumination light. The illumination light is supplied from the rear surface of the liquid crystal display panel module 104 so that the liquid crystal display performs display. The fourth module is a backlight driver 105 for controlling and driving the backlight module 100.
Conventionally, a cold cathode fluorescent lamp (CCFL) has been widely used as a light source of the backlight module. In recent years, however, a light emitting diode (LED) is also used, instead of the CCFL, as a light source of the backlight. The LED is easily controllable in terms of on-off control of the light emitting periods, and the light emission amount thereof may also be controlled with ease by controlling the amount of current. Accordingly, as compared with the CCFL, the LED is capable of attaining low power consumption. Further, the LED is smaller in physical configuration as compared with the CCFL, and hence the light source region to be illuminated by one LED element may be reduced in area. It should be noted that the LED is a point light source, and hence it is necessary to provide an optical member on the circumference of the LED so as to diffuse in plane the light emission amount of the LED so that a uniform luminance may be attained in a planar direction.
FIG. 2 illustrates an example of how the LEDs are connected in a case where the LEDs are used as a backlight. As an example of how the LEDs are connected in a case where the LEDs are used as a backlight, as illustrated in FIG. 2 and as disclosed in JP 4177022 B, a power supply circuit 110 for supplying voltage, a plurality of LEDs 101, and a field-effect transistor (FET) switch 113 which operates as a constant current source for adjusting an amount of current are connected in series with respect to one LED chain, and the FET switch 113 is turned on and off so as to allow a constant current to flow through the LED chain, to thereby turn on and off the LEDs 101. The FET switch 113 is disposed inside a backlight driver IC 111. Further, one backlight driver IC 111 includes therein a plurality of the FET switches 113 so as to respectively control the plurality of LED chains.
Here, the LEDs 101, which are light emitting elements, have a feature in that a degree of voltage drop significantly varies from one element to another. In a case where a plurality of LEDs 101 are connected to one LED chain and the plurality of the LED chains are driven by one backlight driver IC 111 while the LEDs 101 significantly vary from one another in voltage drop, the backlight driver IC 111 consumes power as heat.
In view of the above, as disclosed in JP 2006-245307 A, a protective transistor is connected in series to each of the LED chains, and a base terminal of the protective transistor is supplied with a predetermined voltage, to thereby limit an input voltage input to the backlight driver IC, so as to suppress heat generated in the backlight driver IC.
As disclosed in JP 2006-245307 A, when the protective transistor is inserted with respect to one LED chain, the input voltage to the backlight driver IC may be limited and heat generated in the backlight driver IC itself may be suppressed. However, power consumption results from an ON resistance of the protective transistor itself. Accordingly, power is consumed to the amount corresponding to the ON resistance, regardless of whether or not there is a variation in voltage drop among the LEDs. In the LED, a current of several tens of mA needs to be supplied, which increases the power consumption resulting from the ON resistance, leading to a reduction in power efficiency in the entire module.